Unsaturated acids, such as methacrylic and acrylic acids, acrylonitrile, and the esters of such acids, such as methyl methacrylate, are widely used for the production of corresponding polymers, resins and the like. Various processes and catalysts have been proposed for the conversion of alkanoic acids, such as acetic acid or propionic acid, and formaldehyde to the corresponding unsaturated monocarboxylic acids, e.g., methacrylic acid, by an aldol-type reaction. Generally, the reaction of acid and formaldehyde takes place in the vapor or gas phase while in the presence of a basic or acidic catalyst.
The literature is replete with disclosures of the reaction of aliphatic carboxylic acid compounds with formaldehyde to produce alpha,beta-ethylenically unsaturated aliphatic monocarboxylic acid compounds of one more carbon atom than in the saturated carboxylic acid. For every molecule of alpha,beta-ethylenically unsaturated aliphatic monocarboxylic acid produced there is one molecule of water by-product. It is necessary to separate the alpha,beta-ethylenically unsaturated carboxylic acid compound, formaldehyde and the starting unsaturated carboxylic acid.
In the case of methacrylic acid, this means that the methacrylic acid must be separated from propionic acid, formaldehyde and water. This separation presents several problems since each of the components are water soluble and because propionic acid and methacrylic acid have boiling points that are so close that it is difficult to fractionate one from the other. Further, the separation is complicated by the fact that methacrylic acid has a tendency to homopolymerize and formaldehyde, if water is removed from the system, also has a tendency to homopolymerize. Of the various alpha,beta-ethylenically unsaturated compounds, it is generally recognized that methacrylic acid has one of the greatest tendencies to polymerize and it is extremely difficult to handle at elevated temperatures. In this regard, we have found that the presence of certain reaction by-products greatly increase the propensity of methacrylic acid to homopolymerize. Specifically, alpha-,beta-unsaturated ketones, i.e., ethylisopropenyl ketone and 2,5-dimethylcyclopenten-1-one, have been shown to greatly increase the degree of methacrylic acid homopolymerization. Additionally, methacrylic acid, propionic acid and formaldehyde individually form binary azeotropes with water. The boiling points of the three binary azeotropes are within 1.degree. F. of each other and are thus exceedingly difficult to separate. The following table lists boiling points and weight percentages of binary azeotropes of water and methacrylic acid, propionic acid and formaldehyde at 760 mm Hg.
______________________________________ Wt Wt % B.P. % H.sub.2 O .degree.F. ______________________________________ Methacrylic acid 23.1 76.9 210.7 Propionic acid 17.8 82.2 210.4 Formaldehyde 18.25-21.0 79.0-81.75 210.4 ______________________________________
In somewhat greater detail, the invention relates to a process for an aldol-type condensation of a saturated aliphatic monocarboxylic acid compound and an aldehyde wherein said monocarboxylic acid is propionic acid and said aldehyde is formaldehyde. As is well-known, an aldol-type condensation can be base-catalyzed and is subject to ready dehydration if the .beta.-hydroxyl group is adjacent to an .alpha.-hydrogen atom. The product is an .alpha.,.beta.-unsaturated acid of one more carbon atom than the original unsaturated aliphatic monocarboxylic acid, when the reacting aldehyde is formaldehyde. The reaction using propionic acid and formaldehyde is: EQU CH.sub.3 CH.sub.2 COOH+HCHO.fwdarw.CH.dbd.C(CH.sub.3)COOH+H.sub.2 O
In the prior art a number of methods have been taught to overcome the aforementioned problems. A method for recovery of methacrylic acid from an aqueous effluent obtained by vapor phase condensation of formaldehyde and propionic acid, where the effluent contains unreacted formaldehyde and unreacted propionic acid, is taught in U.S. Pat. No. 4,040,913, wherein the recovery steps include extracting the effluent with an organic solvent capable of azeotroping with propionic acid to obtain an organic phase and an aqueous raffinate; distilling the organic phase to remove 50-100% of the unreacted propionic acid and leaving as bottoms remaining propionic acid and methacrylic acid; distilling the bottoms to obtain methacrylic acid as bottoms; and distilling the aqueous raffinate with an entrainer to obtain dilute aqueous formaldehyde overhead which is further concentrated by distillation. The aqueous raffinate and organic phase are separated by decantation.
Use of selective solvents for recovering unsaturated acids such as acrylic acid and methacrylic acid from aqueous mixtures is taught in the prior art. For example, U.S. Pat. No. 3,414,485 teaches a method of removing water from methacrylic acid in a two-stage process. In the first stage, methacrylic acid is extracted with an organic solvent which forms a minimum-boiling azeotrope with water. Suitable organic solvents include o-, m- and p-xylene, toluene, n-octane, monochlorobenzene, methylamylketone, ligroin and methyl methacrylate monomer. U.S. Pat. No. 3,478,093 teaches use of a lactam having 4 to 7 ring members and a hydrocarbon radical substituent on the nitrogen atom as an extraction solvent to separate methacrylic acid from aqueous mixtures. U.S. Pat. No. 3,781,332 teaches use of a dual mixture containing methyl or ethyl methacrylate and not more than 50% of xylene, ethyl benzene or a mixture thereof. U.S. Pat. No. 4,142,058 teaches use of a mixed solution of methyl methacrylate and toluene to separate methacrylic acid from an aqueous solution containing acetic acid. U.S. Pat. No. 4,147,721 teaches use of methyl n-propyl ketone.
Initially, it was believed by workers in this laboratory that it would be possible to separate methacrylic acid from propionic acid, formaldehyde and water by extraction and fractional distillation using a suitable solvent, such as an aliphatic hydrocarbon of from about 6 to 12 carbon atoms, as is taught in U.S. Pat. No. 4,409,128 or G.B. Pat. No. 2,001,315B. Unfortunately, attempts to implement the separation led to solid paraformaldehyde formation in the column between the feed location and the top of the column resulting in plugging of the column. The separation of these products is, of course, complicated by the fact that the unreacted saturated aliphatic carboxylic acid and formaldehyde must be recovered and returned to the main reactor in order to have an economically attractive process. Accordingly, the formation of solid paraformaldehyde in the column must be manageable from both an operational point of view and an economic point of view. Likewise, it is desirable to minimize the amount of unreacted formaldehyde that must be processed before returning same to the reactor.
The general object of this invention is to provide an improved method of treating the reaction product stream comprising the alpha,beta-ethylenically unsaturated aliphatic monocarboxylic acid compound of one more carbon atom than the starting saturated monocarboxylic acid compound, the saturated monocarboxylic acid compound, water, formaldehyde and by-products. A more specific object of this invention is an improved method of separating methacrylic acid, propionic acid, water, formaldehyde and by-products.
The objects of this invention can be attained by distilling the reaction product of a saturated aliphatic monocarboxylic acid compound and a formaldehyde compound comprising an alpha,beta-ethylenically unsaturated aliphatic monocarboxylic acid compound of one more carbon atom than the starting saturated aliphatic monocarboxylic acid compound, water and by-products, which comprises fractionally distilling said reaction products whereby a sidestream is removed from a location above the location of the feed stream input to the distillation column. In those cases where a distillation column is not in direct contact with a vessel or a reactor containing catalyst and is downstream from the reactor, the location of the sidedraw is located such that at least 10 (wt)% up to 95 (wt)% of the unreacted formaldehyde, and at least 10 (wt)% up to 70 (wt)% of the unreacted propionic acid in the column is removed from the column in the sidestream. The sidestream is composed of no more than 50 (wt)% water plus methacrylic acid. In a preferred method, the reaction products are distilled together with a substantially non-reactive compound capable of acting as a water-entrainer and of breaking a water azeotrope of said saturated aliphatic carboxylic acid compound under conditions whereby (1) a major proportion of the ethylenically unsaturated monocarboxylic acid compound remains in the bottom of the column, (2) a major portion of the water, a portion of the formaldehyde compound and a major portion of the compound capable of acting as a water-entrainer and of breaking or preventing the formation of said azeotrope are removed overhead and (3) a sidestream is removed below the top of the distillation column comprising water, a major portion of formaldehyde and a substantial proportion of saturated aliphatic carboxylic acid.
In the case of the separation of the reaction products of methacrylic acid, propionic acid, formaldehyde and water, we have found that by removing a sidestream below the top of the distillation column, it is possible to recycle a substantial portion of the unreacted formaldehyde and propionic acid to the reactor and avoid the polymerization and plugging of the distillation column by polymerized formaldehyde.
In addition, a major advantage of using a side draw stream and directly recycling this stream to the methacrylic acid synthesis reactor is the reduced costs in the overall process which accrue because unreacted propionic acid and formaldehyde do not have to be separated and individually purified.
In a preferred method of operation, the unreacted propionic acid and formaldehyde are recycled to the inlet ports of the reactor and employed to produce methacrylic acid.